This invention relates generally to tractor loaders and, more particularly, concerns the boom assembly of the loaders.
The conventional arrangement for a loader boom assembly consists of a pair of boom arms which are spaced apart by cross tie members that maintain a uniform distance between the boom arms. The three basic configurations of the boom arms are a solid plate, a box structure formed by single, multiple bend, wraparound sheet and a box structure formed by welding two C-channels.
Although the solid plate and the single piece wraparound boom configurations have the best strength characteristics, the expense of obtaining a plate of proper thickness and size, and the expense of forming the single piece wraparound box have made the C-channel box configuration the most popular among today's manufacturers because of its lower manufacturing cost. The use of this least expensive C-channel box boom structure has presented design difficulties because it includes four bends and a continuous weld between the channels which weaken the structure in the twisting load mode. To overcome this weakness the position setting cross tie members have been extended completely through both channels and have seen welded to both channels to supply the necessary twisting strength to the configuration.
When the cross ties were modified in this manner to perform a dual function, the design change produced a multiplicity of problems for the industry. The first of these problems arose from the modifications in the assembly procedure that were necessitated by the structural change in the boom assembly. The new assembly procedure as set forth in U.S. Pat. No. 3,254,780 issued on June 7, 1966, in which the assembly sequence of welding the C-channels into a box structure, cutting the holes for the boom implement and cross tie member and welding the cross tie members to both boom arms. This assembly sequence increases the cost of manufacturing the boom assembly because it requires a very accurate forming of the individual C-channels and a drilling process to produce the necessary holes.
These other difficulties with this passthrough cross tie configuration involve weaknesses in the actual structure of the assembly. The first of the structural weaknesses is caused by the stress risers which are produced in the C-channels when the cross tie holes are drilled therein. Although the embracing effect of the passthrough cross ties does increase the twisting strength of the boom arms, the holes in the sides of the boom arms cause stress risers which decrease the strength of the assembly in the normal straight lifting mode. The remaining two structural weaknesses are the result of the dirty and damp environment in which the tractor loaders operate. This environment will weaken the passthrough cross ties by exposing the external welds of the cross ties to rust, and will also attack the integrity of the cross tie itself by filling the open ended cross tie with dirt and moisture that once again will weaken the structure through rust.
It is, therefore an object of the present invention to provide a C-channel boom arm which has increased twist strength with minimal effect on the strength of the boom arm in other loading modes while giving a positioning characteristic to the cross tie.
Additionally, it is an object of the present invention to provide a C-channel boom arm with the above increased twist strength of the arm without increasing its susceptibility to the corrosive environment in which it must operate.
Further it is an object of the invention to provide a method of fabricating the above-described boom arm assembly which maximizes the efficiency of the fabrication thereby minimizing the expense of the process.